Generally, an electric activity in a living body is measured in the form of an electrocardiogram, for example, by measuring voltages generated at electrodes attached on a surface of the living body.
For example, Patent Document 1 discloses a method including measuring surface potentials at points on a line of intersection (a closed curve) of a living body and a predetermined plane, and determining an electric potential distribution in a cross section, of the living body, passing through the plane.